EVALUATION OF SHEEP OOCYTES SUBMITTED TO HEAT STRESS INDUCED DURING IN VITRO MATURATION

Abstract

The aim of this work was evaluate the effect of heat stress during oocyte maturation on ovine in vitro embryo production. Ovaries were collected at abattoirs and oocytes retrieved from follicles ranging from 2 and 6 mm in diameter. After selection, all oocytes, in 10 replicates, were placed in in vitro maturation (IVM) during 24 hours. The oocytes were submitted to heat stress of 41º C during 3, 6, 12, 18 and 24 hours and were further transferred to 39º C in order to complete IVM, which was the temperature of maturation of control oocytes. Embryonic development was determined on days 3, 4, 5, and 8 post-fertilization. Embryo evaluation was performed as total cell count by DAPI staining and determination of positive blastomere for apoptosis by the TUNEL assay. We observed that heat stress diminishes (P < 0.05) oocyte maturation capacity in accordance with exposure time of 41º C. In the group of oocytes incubated at 39 ºC, 70.70% matured, while in the groups exposed to heat stress of 41º C, only 45.28%, 35.17%, 12.30%, 9.74% and 4.60% matured, respectively, after 3, 6, 12, 18 and 24 hours of incubation. The duration of exposure to heat stress is inversely proportional (P < 0.05) to embryonic developmental capacity directly proportional (P < 0.05) to the number of blastocysts positive to apoptosis. However, the cleavage rate and embryonic development from 8 to 16 cells and morulae stages were affected by heat stress (P > 0.05) only up to 18 hours of incubation. The results allow the conclusion that heat stress during oocyte in vitro maturation reduces the quantity and quality of ovine embryos produced in vitro determined by the high incidence of apoptosis.

KEYWORDS: blastocyst; IVF; IVM; IVP.